The present invention relates to an intake air amount control apparatus for an internal combustion engine mounted on a vehicle provided with an automatic transmission.
An automatic transmission for a vehicle sets the shifting pattern for the gear ratio in accordance with the maximum power of the engine and the driving characteristics required for the vehicle. According to the shifting pattern, the gear ratio is changed according to, for example, the depression degree of the accelerator pedal and the vehicle speed.
For example, “Manual for Mark II” issued by TOYOTA JIDOSHA KABUSHIKI KAISHA in September, 1996 discloses an automatic transmission that is capable of changing the shifting pattern for the gear ratio according to the operating condition of the vehicle. When giving more priority to fuel economy than acceleration performance of the vehicle, this type of automatic transmission permits the gear ratio to be reduced at a time earlier as compared to the time of shifting up by a conventional automatic transmission. Also, it permits the gear ratio to be reduced at a time later as compared to the time of shifting up by a conventional automatic transmission when giving more priority to acceleration performance than fuel economy. In this manner, the shifting pattern for the gear ratio is switched between a mode for giving priority to fuel economy and a mode for giving priority to acceleration performance, so that the driving characteristic desired by the driver is obtained.
In a vehicle internal combustion engine, intake air amount is adjusted by changing the opening degree of the throttle valve. Decrease in the opening degree of the throttle valve increases pumping loss. This could in turn reduce fuel economy. In this respect, Japanese Laid-Open Patent Publication No. 2001-263015 discloses a variable valve actuation mechanism that changes the maximum lift of intake valves according to the operating state of the engine. The variable valve actuation mechanism is capable of increasing the opening degree of the throttle valve while reducing the maximum lift of the intake valves when the intake air amount required for operating the engine is small, for example, when the engine is idling. Accordingly, pumping loss is reduced during low load operation of the engine. This improves the fuel economy.
While the technique for switching the shifting pattern for the gear ratio is designed for improving the acceleration performance of vehicles, the technique for changing the maximum lift of intake valves using a variable valve actuation mechanism is designed for improving the fuel economy of the vehicle. Since the purposes of the techniques conflict with each other, applying these to an actual vehicle creates problems in terms of vehicle driving characteristics.
That is, when the gear ratio shifting pattern is switched to a mode for giving priority to acceleration performance, if the maximum lift of intake valves is reduced by the variable valve actuation mechanism, it takes a relatively long time for the intake air amount to reach maximum. In this case, the variable valve actuation mechanism is controlled to gradually increasing the maximum lift of the intake valves. The variable valve actuation mechanism however starts operating with a response delay. Thus, despite the fact that the gear ratio shifting pattern has been switched to the mode for giving priority to acceleration performance, the engine cannot produce great torque. The acceleration performance of the vehicle therefore cannot be sufficiently improved.